Multiple site communication systems which comprise a plurality of repeaters and transceivers that are distributed throughout a large geographic region are well known. Many multi-site systems use same-frequency simulcast, i.e., the same communication channel (or carrier frequency) is used by multiple sites throughout the region to simultaneously relay communications to subscriber communication units that are located throughout the multi-site system. This is an efficient frequency reuse technique when the subscribers are routinely located throughout the multi-site system.
The use of comparators, and their associated voting algorithms, within simulcast communication systems is known. In general, a comparator, operably coupled to a plurality of base stations or satellite receivers located in geographically diverse areas, attempts to select or construct a favorable representation of an audio signal given multiple sources of the signal (e.g., the base stations). This is accomplished by comparing the signals received from the signal sources and selecting, from amongst all of the signal sources, portions of the signal having the best signal quality. The selected portions are then reassembled to produce a favorable voted signal frame. The voted signal frame can then be retransmitted by a base station, thereby increasing the probability of good reception (i.e., intelligibly decoded audio) at the signal destination (e.g., a mobile communication device). The signal selected as the best by the comparator is typically distributed therefrom to the transceiver sites for simultaneous re-transmission.
More recently, digital simulcast radio communication systems have been developed. In such systems, digital information is formatted into information frames. Each information frame is made up of a plurality of information packets that are transported through the system infrastructure.
All simulcasted information packets are processed through a comparator which receives inputs from multiple signal sources and selects an input signal source based on predetermined criteria of signal quality. The comparator, by way of a simulcast launch timestamp server coupled thereto (or incorporated therein), then assigns a launch time (launch timestamp) to each information packet received from the selected input signal source, and transmits the information packet over an infrastructure link to at least one of a plurality of base stations, where the information packet is temporarily stored in a buffer. At the assigned launch time, error correcting information is typically added to the information packet and the resulting data packet is transmitted by the base stations.
Simulcast systems which employ absolute launch timestamps require the distribution of a precise time reference--realized by a global positioning system (GPS) receiver or the like--to multiple timestamping resources in a multi-channel simulcast system. Typically, a dedicated comparator is assigned to provide voting to a single radio channel in a multi-channel simulcast system. In such a system, simulcast launch timestamping is performed independently by each comparator.
Timestamping is performed by the launch time stamper. The time stamp normally correlates the received signal with the time it was received (time of arrival). In the present exemplary embodiment, the time stamp represents a pre-calculated launch time for the eventual transmitted signal where the launch time is derived by adding a predetermined offset to the time of arrival. The predetermined offset is a constant that is chosen based on the expected worst case digital network transmission delay between any two sites.
For effective simulcast transmission, each transmitter associated with a given channel must have a replicated copy of the information packet to be transmitted (to the appropriately addressed receivers) in a buffer before the launch time.
The inclusion or assignment of a dedicated precise time reference generating means within each comparator has the effect of eliminating the need to distribute the time reference signal. However, although this scheme eliminates the time reference distribution requirement, it does so at considerable cost and complexity.
Furthermore, because a comparator dedicated to a single radio channel has only a single resource capable of timestamping, each channel has a single (non-redundant) point of failure.
Therefore, a need exists for a redundant centralized timestamping device which can function as a simulcast controller for multiple channels simultaneously.